Power transformer is one of the main elements in the power system. Therefore it is very important to pay sufficient attention to maintenance, diagnostic and lifetime issues. Assessment of power transformers is a complex task from both – technical (condition) and economical (benefits and costs) point of view. The traditional economical tool of a cost budget for each individual element of the organisation does not show the effect on the profit that maintenance has. Economic assessment is dependant on ‘transformer availability factor’ and failure rate of the transformer population, in other words, it’s very important to determine remaining lifetime. Maintaining the technical condition its economical life has a positive effect on the final economic results of the asset. Condition-based maintenance, prognostics and health management, and other types of power transformer maintenance strategies are considered and analyzed in this paper. To understand the role of predictive prognosis, one has to understand the relationship between diagnosis and prognosis capabilities. Diagnostic capabilities traditionally have been applied at or between the initial detection of a system, component, or subcomponent failure and complete system catastrophic failure. The risk, that power transformer will not in working condition by inner defect, decrease, if the operating personnel are trained and able to realize basic moments of these maintenance strategies. It is very important to transformer holder from economical side, especially nowadays.